1. Field of the Invention
The present invention relates to light-emitting diodes (LEDs) having a stripe waveguide structure.
2. Description of the Related Art
As a conventional printing system for a high definition photograph, a digital printer (“Frontier”) equipped with an exposure system has been produced by the present applicant. The exposure system is used to expose a regular photosensitive material by employing a semiconductor laser as a red light source and also employing solid-state excitation lasers (utilizing second harmonic generation (SHG)) as blue and green light sources. In addition, there has been produced a relatively inexpensive exposure system (“Pictography”) for exposing a photosensitive material that generates the colors to cyan, magenta, and yellow, by the use of a semiconductor laser light with two infrared wavelengths and a red wavelength. These exposure systems can perform high-speed light-beam scanning by employing a polygon mirror, etc., because they employ laser light sources. Therefore, they are able to perform printing at high speeds. In addition, it is not necessary for these exposure systems to have a high-speed paper feeder, such as a drum, etc., for scanning; they are capable of utilizing easier paper feed methods, such as those which supply continuous paper.
The former is able to employ regular color paper, which conforms with the sensitivity of the three primary colors, has the most stable characteristics and is of low cost. However, the former is able to use a semiconductor laser for red, but semiconductor lasers for green and blue have not been put to practical use. Because of this, small and high-definition laser light can be provided at relatively low cost by employing a gas laser or solid-state excitation laser (utilizing SHG) as a light source. In addition, since the electro-optic conversion at the initial stage is made by a semiconductor laser, the reliability is enhanced.
The latter, on the other hand, can employ commercially-available semiconductors (e.g., laser emission wavelengths 810 nm, 750 nm, and 680 nm), but it is necessary to employ a special photosensitive material that conforms with these wavelengths. As a result, the latter cannot be widely used for various purposes, and the printing cost increases. In addition, to conform with commercially-available semiconductors, the sensitivity is more shifted to the side of long wavelengths than is usual. Because of this, the durability is generally reduced compared with photosensitive materials for shorter wavelengths, and consequently, special care must be taken in handling. In addition, in such a method, the printer can be more inexpensively manufactured than the case of employing the above-mentioned solid-state laser, but it is necessary to prepare a special photosensitive material that conforms with the emission wavelengths of a semiconductor laser. Because of this, the material cost becomes high and consequently the running cost and the printing cost will be increased.
Therefore, to achieve further cost reduction in these devices, it is extremely important to reduce the cost of the laser light source which is the key device. The cost of red semiconductor lasers are being reduced for their use as a light source for a high-density magneto-optic disk or digital video disk (DVD). However, for blue and green, it is difficult to obtain semiconductor lasers, and in the solid-state lasers currently in use, cost reduction like a semiconductor laser is difficult due to the number of components and the assembly cost.
In addition, as the light source of a conventional digital printer for a silver-chloride photograph, it is ideal to employ a laser light source that emits light in red, green, and blue bands. In the case a material whose sensitivity is high like a general-purpose silver chloride sensitive material is employed, the light intensity on the sensitive material may be a low light output on the order of 1 μW. Therefore, instead of an expensive semiconductor laser, a light source for a printer may be an LED having the advantages of laser light that the spot size (beam diameter) is small and that the radiation angle of the light beam is narrow.
At present, high-brightness blue and green LEDs have been realized with an InGaN-system material (see “The Blue Laser Diode,” S. Nakamura and G. Fasol, Springer, Berlin, 1997). These LEDs, as with other LEDs for red, etc., have a light-emitting surface of a few hundred μm2, and the light is divergent light. Therefore, to constitute a high-definition printer, if the LEDs are employed as light sources to form a spot of a few ten μm in diameter, there is a problem that the light quantity of divergent light which is coupled into the finite aperture of an optical system will become extremely small, and that since an image is formed by a reduction optic system of about ½ to {fraction (1/10)}, the lens is moved away from the light source and therefore the coupling efficiency will be further reduced.
To solve the above-mentioned problem, a semiconductor light-emitting element with an optical waveguide structure having a micro light-emitting region, and an exposure apparatus, have been proposed in Japanese Unexamined Patent Publication No. 11(1999)-074559 by the present inventor. In the semiconductor light-emitting element with a stripe structure disclosed in the above-mentioned publication, it has been found that in a region of complete spontaneous emission light having no stimulated emission, the light beam can be collected to a microspot.
An LED with such a stripe waveguide region has hitherto been fabricated with the same design as that of a semiconductor laser. However, it has become apparent that in an LED, that does not employ a resonator, it is not optimal to utilize the same design as that of a semiconductor laser. Particularly, in semiconductor lasers, the reflectance factor at the resonator end plane and the resonance length are important design factors, because they can be the main cause of the efficiency loss of the resonator. On the other hand, the LED, which has practically no stimulated emission or no stimulated emission at all, is quite different in design from the semiconductor laser, because it does not operate as a resonator, even if it has a similar stripe structure. Furthermore, since spontaneous emission light is not high in wave-guiding efficiency, the light emitted from the rear end portion of the stripe waveguide region is absorbed in the active layer and therefore it is not effectively emitted from the front facet.